Nonketotic hyperglycinemia, is a recessively inherited autosomal disorder of the amino acid metabolism caused by a deficiency in the mitochondrial glycine cleavage system. Neonatal type is the most common form. Infants are usually normal at birth and clinical manifestations such as severe hypotonia, poor feeding, seizures, and lethargy progressing rapidly to a deep coma are seen during the first few days of life. Majority of the affected infants die during the first weeks of life. Those who survive develop severe neurological sequelae. The aim of this case series is to evaluate the clinical features, treatment approaches and short term prognosis of the infants diagnosed with neonatal nonketotic hyperglycinemia during the last 5 years in our department. Data were collected retrospectively from patients' files whose postnatal age at diagnosis varied between 2 to 14 days. All patients were admitted with failure to suck and lethargy, and all had severe hypotonia and decreased newborn reflexes on physical examination. Four patients developed resistant myoclonic seizures and deep apnea requiring mechanical ventilation support. In all patients diagnosis was made based on high plasma and cerebrospinal fluid glycine levels. Genetic study could be performed in only one patient. However enzymatic analysis could not be performed in any patient. All patients demonstrated pathological neuroimaging results, and electroencephalographic abnormalities including multifocal epileptiform abnormalities and "burst supression" patterns in four patients. All patients received low protein diet and drugs reducing plasma glycine levels. Treatment-refractory seizures could only be controlled by levetiracetam in four patients. While two patients died during follow-up, and remaining three patients survived with severe neurological sequels. Physicians should consider nonketotic hyperglycinemia in differential diagnosis in our country where consanguineous marriages are frequent, especially when a newborn healthy for a certain time period develops severe hypotonia, resistant seizures and encephalopathy as detected with routine laboratory tests performed during follow up period.